A hydraulic control system is employed in industrial machineries and automobiles to control and actuate a device hydraulically actuated. For instance, in an automatic transmission of vehicles, a speed ratio thereof is changed hydraulically, and a transmission torque thereof is varied according to a pressure level. For this purpose, the fluid pressure is established by driving an oil pump by a prime mover of the vehicle, and the fluid pressure thus established is regulated to initial pressure called line pressure by a regulator valve. The line pressure thus established is further regulated according to need and delivered to a transmission, a clutch, a brake and etc. Japanese Patent Laid-Open No. 2010-151240 discloses an example of hydraulic control system applied to a belt-type continuously variable transmission. The hydraulic control system taught by Japanese Patent Laid-Open No. 2010-151240 comprises a high pressure hydraulic circuit for controlling the fluid pressure delivered to a pulley on which a belt is applied to change the speed ratio, and a low pressure hydraulic circuit for controlling the pressure of the oil delivered to a torque converter and the oil delivered to a portion to be lubricated. According to the teachings of Japanese Patent Laid-Open No. 2010-151240, the fluid pressure established by the hydraulic pump driven by the engine is regulated to a required pressure in the high pressure hydraulic circuit by a first regulator valve. The fluid pressure thus regulated by the first regulator valve is further lowered by a second regulator valve to be delivered to the torque convertor, and the fluid drained from the first regulator valve is delivered to the portion needs to be lubricated. In addition, an accumulator is arranged in the high pressure hydraulic circuit to accumulate the high fluid pressure applied to the high pressure hydraulic circuit. A solenoid switching valve is connected with an input side of the accumulator to connect the oil pump selectively with the accumulator and the high pressure hydraulic circuit. The electric oil pump for generating high pressure is connected with an input side of the changeover valve.
The above-explained first and the second regulator valves are adapted change the pressure regulating level in accordance with signal pressure from a predetermined solenoid valve. In addition, according to the teachings of Japanese Patent Laid-Open No. 2010-151240, the changeover valve is activated by an electromagnetic force of the solenoid, and the drive and the driven pulleys are individually provided with a feeding solenoid valve and a draining solenoid valve for controlling the fluid pressure delivered thereto. Meanwhile, a hydraulic control system or a hydraulic control circuit for a geared transmission is disclosed in Japanese Patent Laid-Opens Nos. 7-119821 and 8-42677. According to the teachings of Japanese Patent Laid-Opens Nos. 7-119821 and 8-42677, signal pressure of a linear solenoid valve for maintaining a slipping state of a lockup clutch is also used to control an engagement pressure of a brake and back pressure of the accumulator.
In order to deliver large amount of high pressured fluid promptly to the pulleys thereby changing the speed ratio quickly, according to the teachings of Japanese Patent Laid-Opens No. 2010-151240, an electrically controllable solenoid valve is used as the accumulator control valve. Therefore, control response of the fluid pressure delivered to the pulley is improved. However, in the control system taught by Japanese Patent Laid-Open No. 2010-151240, the solenoid valves are used to regulate the fluid pressure, to deliver and discharge the fluid to/from the pulleys, and to connect the accumulator and the oil pump selectively. Thus, a large number of the electrically controlled solenoid valves are used in the control system taught by Japanese Patent Laid-Opens No. 2010-151240, and this will increase a cost of the control system. In addition, a size of the control system has to be enlarged.
Thus, according to the teachings of Japanese Patent Laid-Opens Nos. 7-119821 and 8-42677, the signal pressure used to control the slipping state of the lockup clutch is also used for other kind of controls. Therefore, number of parts such as solenoid valves for establishing the signal pressure to be used for other kind of controls can be reduced. However, according to the teachings of Japanese Patent Laid-Open No. 8-42677, the back pressure of the accumulator is controlled under the situation in which the slipping control of the lockup clutch is not carried out. Likewise, according to the teachings of Japanese Patent Laid-Open No. 7-119821, the engagement pressure of the brake is also controlled under the situation in which the slipping control of the lockup clutch is not carried out. That is, both Japanese Patent Laid-Opens Nos. 7-119821 and 8-42677 disclose techniques to utilize unused solenoid valve for other kind of controls, and teachings of those prior art documents do not relate to a technique to use predetermined devices or parts for multiple applications simultaneously. Therefore, the hydraulic control systems taught by Japanese Patent Laid-Opens Nos. 7-119821 and 8-42677 are required to be further downsized, and manufacturing cost thereof can be further reduced.